The invention relates to a method and a device for discharging an electrical network and to an electrical system.
It is known to use in a motor vehicle an electrical machine which on the one hand is used as a starter and on the other hand as a generator. An electrical machine of this type is also called a starter generator. In this case, the electrical circuit is such that the electrical machine is connected to the onboard power supply network and the battery via an inverter, for example a pulse-controlled inverter. When starting, the electrical machine draws the necessary electrical power from the vehicle battery, wherein the current supplied by the battery is converted in particular into a three-phase alternating current using the pulse-controlled inverter, said current driving the electrical machine which, in this case, is acting as a starter motor. During the start process, the electrical machine, which is normally connected to the crankshaft, therefore brings the internal combustion engine of the vehicle up to the required starting rotational speed. After the starting process, the electrical machine is driven by the internal combustion engine and acts as a generator. The generator therefore generates the electrical energy necessary for supplying the onboard power supply network or charging the battery. The output voltage of the electrical machine operating as a generator is regulated to predefinable voltage values using a voltage regulator, for example by influencing the field current, and is rectified using the pulse-controlled inverter. For intermediate storage or transferring electrical energy, electrical systems of this type having electrical machine, generator, pulse-controlled inverter and battery usually also have an intermediate-circuit capacitor. In this case, the intermediate-circuit capacitor charges up to at least the voltage supplied by the electrical machine.
If an electrical system of this type is used in connection with a traction network at a higher voltage, at least one more DC DC converter, which is connected such that it lies between the fraction network at a higher voltage and the onboard power supply network at low voltage, is usually present. In this case, the traction network supplies with electrical energy an electrical machine which acts as the vehicle drive. An example of an arrangement of this type is described in DE 199 03 427 A1.
Particularly in a traction network at a higher voltage, in which the intermediate-circuit capacitor is also at a higher voltage, it must be ensured that the intermediate-circuit capacitor is discharged in a relatively short time after switch-off. For example, it is required that the voltage across the intermediate-circuit capacitor has dropped to below 60 volts within 5 seconds of the pulse-controlled inverter being switched off. In the case of systems used at present, the discharging of the intermediate-circuit capacitor takes place via a resistor connected in parallel, by means of which the intermediate-circuit capacitor or, optionally, the intermediate-circuit capacitors is/are discharged, wherein it is also possible for said resistor to be connected in using a relay when the pulse-controlled inverter is switched off. A discharge circuit is known from DE 10 2004 057 693, in which the traction network is discharged by means of the DC DC converter, which is arranged between the traction network and the onboard power supply network, and, at the same time, the electrical energy is transferred into the onboard power supply network.
The considerations and implementations to date use exclusively static considerations. In this case, only static, that is to say permanently occurring, voltages are taken into consideration. For temporary, that is to say dynamic, considerations however, some exceptions apply, of which the so-called load dump pulse is used in the method presented here. The term “load dump” is understood to mean the occurrence of voltage spikes in the motor-vehicle onboard power supply network. These occur at the instant at which high-power loads of the onboard power supply network are switched off and the loading of the onboard power supply network suddenly decreases. Said voltage spikes or voltage pulses, that is to say dynamically maximum voltages, occur in principle in every onboard power supply network and must be afforded tolerance by each control device according to the specifications.
By way of example, such voltage spikes are permissible if they do not last longer than 400 ms and do not exceed an electrical voltage of 35 volts. The permissibility of voltage spikes of this type in the onboard power supply network is used in the invention described here.